Spark plug



July 23, 1940. F. D. SPRAGUE 2,208,708

SPARK PLUG Filed June 2, 1939 INVENTOR. FRANK DESMOND SPRAGUE ATTORNEYS Patented July 23, i49

guests Frank Desmond SnragueyNew Canaan,

Sprague Specialties Qompany,

, a corporation of Massaassignor to North Adams, Mass. chusetts Conn Application June 2, 1939, Serial No. 277,073

10 @iaims.

The present invention relates to improvements in spark plugs, and particularly in spark plugs for high-compression airplane engines which are shielded to prevent radio interference.

Modern high-compression airplane engines onerate at increasingly high temperatures, which puts increasingly severe demands on the spark plugs of such engines, and particularly on the insulate it from the shell, and of mica washers disposed perpendicularly to the central electrode and which provide mechanical and thermal protection of the sleeve against the scorching and corrosive action of the burning fuel. This cone struction, however, has shortcomings as satis= factory performance of the insulation depends on the gas-tightness of .the washers, which tends to deteriorate in operation. As a result of this, corona discharge and sparkovers are likely to occur between the center electrode and the shell, particularly at take-off" when the engine is cold and is operating at maximum cylinder pressure and at maximum spark-gap voltage.

Furthermore, mica itself has certain shortcom-.

ings in such use. For example, it is attacked by the combustion products of fuels comprising anti-knock compounds-now increasingly used-which results in its. gradual disintegration in operation. Disintegration of mice is also likely to take place because the high temperature to .which mica is subjected in operation is close to the critical temperature at which it water of crysta v ation.

Also, due to the low expansion coemcient of mica as compared with steel, the mica washers are liable to loosen. j

A further drawback of present airplane spark-, plug constructions and the use oi mica insulators is that the heat is very poorly conducted away from the spark-plug tip, sume frequently temperatures pf 700 to 850 C., brin ing about its deterioration and also undesired'preiinition in the engine.

insulators, it has been proposed and have various .afiected with increasing gives up its causing it to as- 1 Such shortcomings of the present-day airplane spark plugs have proven to be a serious limiting factor which retards the development of airplane engines with still higher compression ratios.

To overcome the limitations imposed by mica.

to use in airplane spark plugs, insulators inade of highly reiractory metal oxides, such as aluminum-, magnesium-, or beryllium-oxide and similar in shape to the porcelain cores used for automobile spark plugs. Such cores can be made by artificially imparting tothese oxides plasticity,for example, pulverizing the oxide and mixing it with a preferably volatile plasticizer,-forming the plasticized oxides into the desired shape core and sintering the so-formed core.

Such sintered refractory oxides have a micro crystalline structure devoid of any glassy phase properties which are particu= larly useful for insulators in spark plugs. For example, their electrical resistance is not greatly temperatures, and cores made therewith have great mechanical strength, immunity to attack by ethyl leaded and other "anti-knock compounded fuels and the combustion products of such fuels, non-porosity, and high thermal conductivity. Notwithstanding the enumerated desirable properties, sintered oxide insulators have found but scant application for spark plugs of airplane motors, and have of same. because of their good heat conductivity, carry a large portion of the heat developed in the combustion chamber to the upper portion of the spark plug, which heat if the spark plug is of the shielded type cannot be properly dissipated. As a result thereof the insulation of the high-voltage cable at the place where the cable is connected to the spark plug, rapidly chars and deteriorates causing short circuits and consequent misfiring oi the engine. 1.

It is therefore an object :of my invention to provide a novel spark plug construction which permits iull utilization of the advantages of high- 1y refractory oxide cores without their disadvanw tages. 1

Another object; of my invention is to provide a novel construction for spark plugs of the type shielded to prevent radio interference.

A further object of my invention is to provide novel means to dissipate the heat from spark plugs.

These and further. objectsofmy invention will appear as thespecification progresses. g H

' been found unsuitable for shielded spark plugs This is due to the fact that such cores,

In accordance with my invention, I provide around the central electrode of the spark plug an insulating core of a good heat-conducting and highly refractory material, and provide for a heat by-pass by means of which substantially all of the heat transmitted by the combustion gases to the lower portion of the core is passed to the grounded spark plug shell rather than being transferred through the core to the upper shielded portion of the spark plug. More specifically, I use a core of sintered aluminum-, magnesium-, or beryllium-oxide and of a shape similar to the porcelain cores used in automobile spark plugs, whereby in a cylindrical space formed between the core and the shell I provide a low melting point metal or alloy which is liquid at the temperature which it assumes in operation, and constitutes the above referred to heat by-p'ass from the core to the shell. Suitable materials for this purpose are lead, lead and tin alloys and in general metallic substances havin a melting point which is below the minimum operating temperature of the portion of the core about which the heat by-pass is disposed.

In general, I prefer to lengthen the cylindrical middle portion of the core and also to increase the radial space between it and the shell beyond that customary in present day spark plugs.

By properly dimensioning the cross-sectional area of the liquid metallic by-pass, the temperature distribution in the spark plug can be accurately controlled and the heat which is transferred from the combustion chamber to the upper portion of the spark plug reduced to a negligible amount.

My invention wlllbe further described with reference to the appended drawing forming part of the specification, and in which,

Figure l is a vertical section of a non-separable type radio-shielded spark plug made inaccordance with my invention;

,Fig. 2 is a diagram illustrating the general paths of the heat flow and heat dissipation in the spark plugs of my invention.

Referring to Fig. 1, the spark plug there shown has a metal shell Ill which comprises a hexagonal nut portion H having a bore 40 and a threaded portion l2 having a smaller bore 4|, the portion [2 being adapted to be screwed into the combustion chamber of the engine (not shown). At the transition from the bore 40 to bore 4| a beveled shoulder 42 is ilormed.

Disposed within the shell l0 and upwardly extending therefrom is a core IQ of insulating mat--- terial, preferably of'sintered aluminum oxide or like highly refractory and good heat-conductin material.

The core l6 comprises a cylindrical middle portion 43 and a lower and an upper tapered portion 49 and 41 respectively, the portion 41 extending into a cylindrical portion 48 which projects beyond the shell l0.

The central portion 43 of insulator f6 has a smaller diameter than has the bore 40, this resulting in an annular space 19 between the shell in and the portion 43.

At their junction with portion 43 the tapered portions 41 and 49 are rounded as shown at 50 and 5|. Against the shoulders 50 and SI, press annular conical gaskets l1 and I8 made of suitable material, such as soft electrolytic copper, which firmly position the core i6. The lower gasket l8 thereby presses with its bottom face.

against the shoulder 42, whereas the upper from the shield asosnoe gasket with its top face presses against shoulder M of the shield l3 later to be described. The gaskets ii and I8 serve as mechanical protection for the core it as well as a gas seal, and also assist in the transmission of the heat from the core 86 to the shell in.

Disposed within an axial bore 52 of core i6 is a rod 2i of suitable metal, such as pure nickel, the lower end or" which carries a frustroconical shaped sparking tip 22 abutting with its base against the end face of tapered core portion it. The tip 22 is made of a metal which is highly resistant to corrosion at high temperatures, for example, of stainless steel. The tip 22 is secured to the rod 2i, preferably by welding, so as to provide for a good heat conducting joint between the two, such welding preferably to take place in a reducing atmosphere, for example, in hydrogen.

To prevent longitudinal displacement of the rod 2i within the core IS, an insert 23 is embedded in the core extension i8 and threadedly engages the free end of the rod 2!.

To insure a gas-tight fit of the rod 2| in the core 16 it is, before its assembly with the core, coated with a high-temperature-resistant ceramic cement 53.

Instead of providing a separate insert embedded in the core is, the rod 2| itself may be cast in the axial bore 52.

The second sparking electrode 24 is disposed in the slightly enlarged lower end 54 of bore 4i, and consists of a non-corrosive refractory metal or alloy for instance of nickel-barium alloy. Electrode M is press-fitted within the bore 54,

and to insure its perfect thermal contact with shell portion l2, it is preferably hydrogen-brazed thereto. The electrode 24 may be of the conventional four-prong type whereby through proper shaping of its prongs and their proper spacing relative to the sparking tip 22, such troubles as may arise from capillary collection of oil or water are minimized and the sparking surfaces can be easily cleaned by a sandblast.

To shield the spark plug from causing radio interference, I surround the upper portion of the core I6 with a tubular shield l3 which extends in'to the bore 40 of the shield l0, and as already stated abuts with its lower shouldered end 44 against the upper sealing gasket H. To

der 44 and also serves to improve heat transfer l3 to the shell l0. Sleeve I5 is made of a good heat-conducting metal, such as copper.

The cable connector, which connects a hightension armored cable 25 to the spark plug, comprises an elbow 28 provided at its free end with a threaded gland socket 26 in which is disposed a soft lead tapered sleeve 21. The other end of the elbow 28 is secured to the shield l3 by means of a flanged'sleeve 31 coupled with an internally-threaded socket 29, the latter engaging a threaded extension 38 of the shield i3.

The cable connector and cable terminal portion of the spark plug are so designed and dimentioned as to permit free interchangeability of the spark plug of my invention with standard spark plugs.

Disposed within the shield i3 and extending from its bottom shoulder 44 to its top shoulder 55 is a. cylindrical mica liner formed of a to each other and to spirally-wound strip of mica. Within the upper portion of the liner 36 and abutting against shoulder 55 is a collar 38 which in its turn surrounds a sleeve 30, the latter extending into and beyond the threaded extension 38 of shield i3. Sleeve 30 and collar 3! are preferably of linen-base laminated Bakelite and are secured the liner 34 by means of a Bakelite cement or other suitable binder.

Disposed within thelower end of the sleeve 30 is a conical brass cup 32 provided with a downward-extending tubular portion 55 into which are secured the strands of the cable 25. Electrical connection between the cup 32 and the center rod 2| is made by means of a conical coil spring 33 which surrounds the extension t and is compressedly disposed between the cup 32 and the fiat enlarged head 57 of the insert 23. The mica liner 35, which covers the entire exposed innersurface of the shield 43, prevents corona discharge and flashovers between the shield 53 and the contact spring 33, respectively the exposed top 51 of insert 23'.

To divert the heat'fiow and prevent the heat from passing from the lower portion of the spark plug to its upper portion via the core it, I provide in accordance with the invention, in parallel to the central portion 63 of the core it, a low-resistant heat path. For this purpose I provide in the annular space it a low meltingpoint metal or alloy 35, for example, lead, or a lead-tin alloy which is liquid at the temperature which it assumes in the normal operation of the spark plug. a Low melting point metals other than the above are also suitable for the purposes of the invention, for example, mercury. However, in this case the gaskets ii and it are required to be of a metal which does not form an amalgam with mercury for example, of dead soft steel or the like.

To permit heat expansion of liquid metal 35, I provide for an expansion space 38 on top thereof. In general, I prefer to give the central portion 413 a greater length and to the space it a greater radial width than that found in stand ard spark plugs, to thereby increase the efi'ect of the low heat-resistant by-pass. I have found that this by-pass permits substantially accurate predetermination of the temperature at.the various points of the spark plug, and'that its proper dimensioning makes it possible to pass through the by-pass to the shell it practically all of the heat absorbed by the lower portion of core i6, thereby maintaining the upper portion of the spark plug substantially ate. temperature, well below the charring temperature of the cable insulation.

To insure a gas tight seal between the spark plug and the cylinder head there is disposed be tween the two a copper gasket 38 which also facilitates heat transfer from the shell it to the cylinder head (not shown). To increase this heat flow, I prefer to use a. solid copper gasket.

Referring to the heat flow diagram (Fig. 2) in which the curved arrows indicate the paths of heat flow, it should be noted that the surface temperature of that part of the core it which is exposed to the combustion gases is substantially uniform and that all oi the heat absorbed by the core portion 59 and the tip 22 passes upwardly. One portion of this heat without reaching the center portion 53 passes to the shell lb through the gasket it. However, the major portion of the heat reaches the center portion I asoarce 3 and is mainly dissipated through the heat by-pass, i. e., through the metallic liquid 35 to the shell it. Such heat as may remain absorbed in the core will pass to the shell through gasket ii and the heat 'fiow which reaches the upper portion of the spark plug is negligible.

The heat transferred from the spark plug core to the shell i0 is given up directly to the surrounding air and through the gasket 36 to the cylinder head 58 of the engine.

The spark plug described hereabove is of the non-separable type. My invention is equally applicable to separable types of spark plugs, in which case the shield and the shell instead of being fixedly connected, are threadedly engaged with each other. This permits disassembling of the spark plug for cleaning and adjustment.

My invention is also usable for the so-called short-reach the lower tapered portion of the core and the threaded portion of the shell are correspondingly shortened.

Thus while I have described my invention in connection with a specific example, I do not wish to be limited thereof will occur to What" I claim is:

l. A spark plug comprising a shell of conducting material,- a center electrode and a second electrode, an insulator disposed between the those skilled in the art.

thereto as obvious modifications type of spark plug, in which case r shell and the center electrode, and means to improve the heat transfer from the insulator to the shell, said means comprising a metallic fluid disposed between said insulator and said shell.

2. A spark, plug comprising a metal shell, an insulating core positioned within said shell, a center electrode supported by said insulator, a second electrode, and means to facilitate heat flow from the core to the shell, said means comprising a metallic fluid disposed between said core and said shell.

3. In a spark plug comprising a shell of conducting material, a center electrode and an in sulator of highly heat-conductive material disposed between said shell and said electrode, heat transfer from the insulator to said shell, said means comprising a metal of low melting temperature disposed between said insulator and said shell.

4. In a spark plug comprising a shell of conducting material, a center electrode, an insulator having a micro-crystalline structure devoid of any glassy phase concentrically positioned within said shell around said electrode, means to facilitate the transfer of heat from said insulator, said means comprising a metal or low melting temperature disposed between said insulator and said shell.

5. In a spark plug, electrode, and two concentrical sleeves disposed between the electrode and the shell, one of said sleeves being of an insulating material and the other consisting oi a metal which liqueiies at the temperature it assumes in the operation of the spark plug.

6. ii. spark plug for high-duty airplane engines comprising a metal shell, a grounded electrode attached to said shell, an insulator spacially disposed in said'shell, a center electrode disposed in said insulator and two spaced gaskets positioning said insulator in said shell, and a heat conducting filling within thespace formed between the shell, the insulator and the gaskets to increase the rate of heat flow from the in- Sulator to the shell.

'7. In combination, a spark plug comprising a shell of conducting material, a center electrode and another electrode, an insulator disposed between the center electrode and shell and spaced from said shell, an insulated high-tension conductor, terminal means for connecting said conductor to said center electrode, and means to center electrode and a grounded electrode, a onepiece insulator disposed around said center electrode, an upward-extending metallic sleeve surrounding the upper portion of said insulator, and a low-melting temperature metal mass disposed within said shell around the lower portion of said insulator.

10. a spark plug comprising a shell of conducting material, a center electrode, a second electrode, an insulator spacially positioned within said shell, an upper and a lower gasket spacially separated and disposed about the middle portion of said insulator, and a heat-conducting filling within the space formed between the insulator, the shell and the two gaskets respectively to substantially prevent the heat flow in the insulator from reaching its portion extending above the upper gasket.

FRANK DESMOND SPRAGUE. 

